VSI-type mineral breakers operate as high-speed “rock pumps”. The receipt, acceleration and discharge of the rock feed introduced to this type of rock crusher passes through a rotating impeller more accurately described as a rock-lined rotor. This rotor is supported in the machine by a main shaft 12 which is held and turns in a bearing cartridge assembly 14, as shown in FIGS. 1-4, in a housing (not illustrated) centered within the machine. The rotating shaft 12 imparts torque onto the spinning impeller or rotor 44. The initial point of impact for the incoming rock mineral feed is the center of the rock-lined rotor directly below which is a mechanical connection between the rotor 44 and the shaft 12.
A popular method of affixing the rotor 44 to the shaft 12 is by the use of a taper lock type of arrangement in which a tapered outer surface 16 of a taper lock 18 and a cooperating tapered inner surface 20 of a rotor boss 22 are drawn together using a top plate 24 and several bolts 26, 28. See FIGS. 2 and 3. Commonly, the taper lock 18 is installed on and around the upper end of shaft 12. Then the rotor boss 22, which is very firmly attached to the rotor 44, is lowered over and around taper lock 18. Top plate 24 is then secured to rotor boss 22 with a first set of bolts 26 which pass through outer apertures 27 in top plate 24 and are threaded into bolt holes 46 in rotor boss 22. A second set of bolts 28 passes through inner apertures 29 in top plate 24 and is threaded into bolt holes 48 in taper lock 18. As the second set of bolts 28 are tightened, top plate 24 and rotor boss 22 are drawn downward towards taper lock 18. When properly tightened, the bolts 26, 28 cause a sliding interference fit between the outer surface 16 of the taper lock 18 and the inner surface 20 of the rotor boss 22. Since taper lock 18 is provided with a longitudinal split 30, as the rotor boss 22 is forced downward, the circumference of taper lock 18 is reduced allowing the taper lock to be physically compressed around the shaft 12. A taper lock fitting thus establishes maximum surface contact between the adjoining parts and achieves a high-pressure, compressed, non-slipping joint through which driving torque is transferred from the shaft 12 to the rotor 44. In addition to providing a strong mechanical joint between the taper lock 18 and the rotor boss 22, use of the taper lock joint allows for easy disassembly of the parts by loosening the bolts which draw the tapered surfaces 16, 20 of the taper lock 18 and the rotor boss 22 together. Thereafter, a small amount of axial movement relieves compression at the tapered surfaces.
A conventional key system acts as a backup to minimize or eliminate any rotational slipping between the parts, ensuring that all the components rotate as one. The taper lock 18 is keyed to the shaft 12 using a longitudinal keyway 32 in the shaft 12 into which is fitted a key 34. There is a mating keyway 36 in the bore 38 of the taper lock 18 which matches and slides over key 34. This forms a positive mechanical connection between the shaft 12 and the impeller or rotor 44. See FIGS. 1-4.
While the conventional taper lock-and-keyway design is effective and generally reliable, it is not ideal for application in a VSI-type mineral breaker where extensive vibrational forces and unpredictable shock loadings routinely occur. Due to manufacturing tolerances and variances, weaknesses can develop that undermine the system. Minute differences between the exterior surface of the shaft and the interior surface of the taper lock lead to “fretting,” the microscopic movement of material under high pressure. Poorly machined surfaces can lead to “notches” in the shaft, along the shaft keyway, or in the taper lock bore. As the shaft is typically a hardened steel alloy, it is vulnerable to the phenomena of “notch sensitivity.” This works similarly to the etching of glass wherein a small imperfection in the material may become the focal point for cracking and part failure. Extended use can result in pitting and poor surface conditions. Finally, experience has shown that a high proportion of shaft failures occur in that portion of the shaft adjacent the bottom of the taper lock where a bending moment is formed by the collective weight of the taper lock 18, rotor boss 22, and rotor 44 resting on the shaft 12. In concert, these irregularities can cause unique loading conditions and stress concentrations which may result in shaft failure. One of the factors that can accelerate failure of the shaft is the introduction of heat 30° F. or more above ambient temperature as the result of drying mineral feed entering the breaker. This added temperature causes the rotor boss 22 to expand resulting in a relaxation of the interference fit in the taper lock joint and consequent loosening of the taper lock's grip on the shaft, frequently leading to fretting of the contact surfaces.
In the normal operation of a VSI-type mineral breaker, the impeller or rotor is routinely removed and re-installed for purposes of maintenance. In some instances, multiple impellers or rotors may be applied to the same shaft and taper lock. All of this removal and re-installation distresses the parts of the taper lock assembly, especially the main shaft, with the result that, as the VSI mineral breaker ages, the main shaft becomes more vulnerable.